This invention relates to the art of nuclear reactors and has particular relationship to the core-support structure of nuclear reactors. A nuclear reactor includes a pressure vessel into which a heat-transfer fluid, typically liquid sodium for fast breeder reactors or pressurized or boiling water for more conventional commercial reactors is pumped under pressure. The fluid flows through the core and is heated; the hot fluid emerges from the vessel and the heat flows via mechanically separated primary and secondary loops to electrical-power generating equipment. Within the vessel there is supporting structure for the core components. Typically for a liquid metal cooled, fast breeder nuclear reactor which generates more fissile fuel than it utilizes, these components include fuel-rod bundles or assemblies, control-rod assemblies, blanket fertile-material or fertile-rod assemblies and removable radial shielding assemblies. The expression "core assemblies" or "core component assemblies" or the word "assembly", when used in this application with reference to components of the core, means one or more types of these assemblies. The core-support structure serves the purposes of locating, supporting, distributing coolant to, and providing axial and radial restraint for, these assemblies.
These assemblies and their supports are composed of material which has a higher specific gravity than the fluid and in the absence of other forces, the core assemblies and their supports would be held in the pressure vessel by gravity. However, there are forces tending to counteract the weights of the assemblies and their support. The heat-transfer fluid is injected under pressure in the lower parts of the assemblies and is distributed to, and guided along, the assemblies. As the fluid flows along the assemblies there is a pressure drop along the fluid so that the fluid which emerges from the core is at a substantially lower pressure than the entering fluid and the resulting differential in pressure counteracts the gravity forces.
The practice, in accordance with the teachings of the prior art, has been to secure the supports for the core assemblies to the supporting structure. Typically a so-called "core-basket" concept was applied in a test facility of substantially smaller dimensions than a reactor serving to produce substantial power. A large number, in one typical case, one hundred fifty seven receptacles for the assemblies are welded to upper and lower plates of the core basket. These plates are joined by a cylindrical skirt and the whole welded structure including the assemblies forms a welded unit which is seated in the pressure vessel with fluid seals interposed between the skirt and the core support structure cavity. The seal is typically a piston ring of INCONEL nickel-chromium alloy or like alloy having good fretting and wear characteristics when used in sliding contact with type 304 stainless steel, of which the vessel is typically composed, in liquid sodium. The fluid is injected at the periphery of the skirt between upper and lower seals; that is, laterally of the receptacles into which the core assemblies are inserted. A welded assembly as just described is costly even for a test facility. There are hundreds of welds each of which must be free of defects. So costly an assembly cannot be scrapped because there is an imperfect weld; the weld must be repaired. Because of the radial flow of the fluid, the distribution of the fluid among the assemblies may not be uniform; inner assemblies receive fluid at a lower pressure than those on the outside. For a reactor capable of producing substantial power, typically 975 megawatts at the reactor and about 300 megawatts at the electrical generators, these difficulties are multiplied. Such a reactor would require a support unit, typically about 100 inches in diameter with several hundred receptacles, those for the fertile fuel differing from each other and different from those for the fissile fuel. The making of a satisfactory large diameter piston-ring seal presents formidable problems.
In another prior-art reactor the core assemblies are supported in receptacles which are bolted to a supporting structure. The removal of receptacles from such apparatus for replacement or repair presents formidable difficulty. As a rule the reactor cannot be shut down and the vessel drained when repair or replacement is demanded. To remove a defective receptacle the bolts which are submerged under the fluid (liquid sodium) must be removed by remote manipulators, a difficult task.
It is an object of this invention to overcome the above-described difficulties and to provide a nuclear reactor, particularly a reactor of the sodium cooled, fast breeder type, capable of producing substantial power, whose core-supporting structure shall not impose the formidable structural engineering demands of prior-art structures, which shall be capable of being produced at reasonable cost, substantially lower than the cost of prior-art structure and which shall at the same time improve the distribution of cooling fluid to the core component assemblies. It is also an object of this invention to provide an assembly for supporting the core component assemblies of a nuclear reactor by whose use the above-described disadvantages of the prior art shall be overcome.